1. Field of the Invention
The present invention relates to a semiconductor pressure sensor and a method for manufacturing the same. Particularly, the present invention concerns an improved semiconductor pressure sensor including a diaphragm formed on a semiconductor substrate surface using a film forming technique and an improved process of making such a semiconductor pressure sensor.
2. Description of the Related Art
FIG. 12 shows a semiconductor pressure sensor constructed in accordance with the prior art.
Such a semiconductor pressure sensor comprises a semiconductor substrate 10 and a vanishable film 12 formed on the major surface of the substrate at the pressure receiving region, the vanishable film having a preselected plane configuration and an isotropic etching property. The major surface of the semiconductor substrate 10 also includes an insulation diaphragm film 14 formed thereon over the vanishable film 12 through the entire area of the major surface, the insulation diaphragm being made of etching-resistant material. The diaphragm film 14 includes at least one strain gauge 16 located in place on the pressure receiving region thereof. The diaphragm film and strain gauge 14, 16 are coated with an insulation passivation film 18 of etching-resistant material. The insulation passivation film 18 includes contact holes 20 formed therethrough at positions opposed to the ends of the strain gauge 16. The opposite ends of the strain gauge 16 are connected to electrodes 22 through the contact holes 20.
The semiconductor pressure sensor further includes at least one etching solution inlet port 24 formed in place at the pressure receiving region thereof, the inlet port 24 extending from the outer surface of the pressure sensor through the insulation passivation film 18 and the diaphragm film 14 to the vanishable film 12. When the etching solution is fed into the pressure sensor through the inlet port 24, the vanishable film 12 and a portion of the substrate 10 can be dissolved off by the etching solution. In such a manner, a reference pressure chamber 26 will be defined by the substrate 10 and the diaphragm film 14.
If such a pressure sensor is used as an absolute pressure measurement type sensor, all the etching liquid inlet ports 24 are sealingly closed by sealing members 24a while maintaining the reference pressure chamber 26 vacuum.
In such an arrangement, a movable diaphragm 14a is provided by the diaphragm film 14 located on the upper side of the reference pressure chamber 26.
When a pressure is applied to the pressure sensor, the movable diaphragm 14a is deflected in direct proportion to the magnitude of the applied pressure. The deflection in the movable diaphragm 14a causes the strain gauge 16 to vary its resistance. If a detection signal created in the deflected strain gauge 16 is taken out through the electrodes 22, that signal can be used to determine the absolute pressure applied to the front face of the movable diaphragm 14a.
However, the prior art has the following problems:
(a) The prior art forms a semiconductor pressure sensor by first machining the vanishable film 12 of polycrystal silicon into a desired configuration and then forming the diaphragm film 14 of silicon nitride over the vanishable film 12 on the substrate 10, the diaphragm 14 having an etching-resistant property.
Thus, the peripheral portion of the movable diaphragm 14a will be supported by the substrate 10 through a stepped structure A as shown by dashed line in FIG. 12. If a pressure is applied to the upper face of the movable diaphragm 14a, the latter cannot resist the pressure strongly and be yet supported under the same condition as it was supported before the pressure was applied to the diaphragm 14a. Therefore, the accuracy of measurement in the strain gauge 16 may be degraded.
If such a semiconductor pressure sensor is repeatedly used, the stepped structure A in the movable diaphragm 14a is subject to fatigue. It is thus difficult to use the semiconductor pressure sensor for a prolonged time period with an invariable accuracy.
(b) In the prior art pressure sensor as shown in FIG. 12, the reference pressure chamber 26 is formed by dissolving a part of the substrate 10 and the vanishable film 12.
On the contrary, the inventors have studied whether the reference pressure chamber 26 Can be formed only by dissolving the vanishable film 12 without dissolving any part of the substrate 10. As a result, the inventors have found that the above matter can be attained by forming an insulation film 28 of etching-resistant material on the surface of the semiconductor substrate 10 and then forming the vanishable film 12 and the diaphragm 14 sequentially over the insulation film 28, as shown by broken line in FIG. 12.
If the diaphragm film 14 of silicon nitride is formed on the semiconductor substrate 10 as made of silicon by the use of the vacuum CVD technique, however, an internal tensile stress will be produced due to the differential thermal expansion between the silicon nitride diaphragm film 14 and the silicon substrate 10. In such a case, if the vanishable film 12 between the silicon nitride diaphragm film 14 and the silicon substrate 10 is dissolved off to form the reference pressure chamber 26, a bending moment will be created at the peripheral stepped portion A of the movable diaphragm 14a, leading to any deformation of the movable diaphragm 14a toward the semiconductor substrate 10.
Where the reference pressure chamber 26 is formed only by dissolving the vanishable film 12, therefore, the movable diaphragm 14a may be flexed against the substrate into contact with the insulation film 28 due to the bending moment, depending on the thickness in the vanishable film 12 and movable diaphragm 14a. Thus, the semiconductor pressure sensor may function improperly.